Yes, you may associate yourself with Airlines like Indigo, you regularly recruit Crew below 21, followed by Training and Deputation on Routes.

However I would suggest you should pursue your Graduation (B.Sc./BA/B.HM/B.Com/BBA) until 21, after you Pass Out School at 18.

Doing a Certificate Course from Certain Professional Flight Hospitality Training Academy like Frankfinn also increases you chances in getting selected with Airlines.